Bloom's syndrome (BS) is an autosomal recessive syndrome characterized by growth retardation, sunlight sensitivity and early onset of various forms of cancer. The diagnostic feature of BS is an increased number of sister chromatid exchanges (SCEs), which is indicative of a deregulation of homologous recombination (HR). Even though HR plays a pivotal role in the repair of damaged chromosomes and helps restart stalled DNA replication forks, untimely or inappropriate HR events can be deleterious. For example, the formation of crossover products during HR can lead to a loss of heterozygosity, and ectopic HR events can cause chromosome translocations. For this reason, cells have evolved several mechanisms to monitor and regulate HR events. Results from studies conducted in the Hickson laboratory and ours have found that BLM works in conjunction with topoisomerase Ilia (Topo Ilia) and the newly identified protein BLAP75 to mediate the dissolution of the double Holliday junction (DHJ) to form exclusively non-crossover recombinants. The goal of this research project is to reconstitute the BLM-Topo llla-BLAP75 (BTB) complex and perform a comprehensive analysis of its activity using multiple DNA substrates that model HR intermediates. In addition, the contributions of BLAP75-mediated protein-protein interactions and DNA binding and the DNA relaxation activity of Topo Ilia toward the biochemical functions of the BTB complex will be determined in biochemical assays and in cells. These studies should shed light on a major pathway of HR regulation that has strong relevance to genome maintenance and cancer avoidance. Relevance to Public Health: Bloom's syndrome (BS) is an inherited disorder that predisposes patients to cancer at a young age. Here, I propose studies that will help elucidate the mechanism of action of BLM, the protein mutated in BS. The results from my research endeavors will be important for deciphering the BLM-dependent mechanism of homologous recombination control and will provide a molecular explanation for the genome instability and cancer susceptibility in BS patients. [unreadable] [unreadable] [unreadable]